The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for regulating currents. Merely by way of example, the invention has been applied to regulate currents flowing through light-emitting-diodes (LEDs). But it would be recognized that the invention has a much broader range of applicability.
Light emitting diodes (LEDs) have been widely used in various electronics applications. LEDs have been recognized for their significant advantages over other lighting sources, such as incandescent bulbs. For example, such advantages include high efficiency and long lifetime. But, challenges remain for low-power LED applications, such as poor accuracy of current, low efficiency of light conversion, and large size of printed-circuit-board (PCB). Poor accuracy of current usually decreases the lifetime of LEDs, and low efficiency of light conversion often increases heat generation, which may also reduce the lifetime of LEDs.
FIG. 1 is a simplified conventional diagram showing a system for driving LEDs. The system 100 includes a system controller 102, a snubber circuit 116, a transformer 118, a rectifying diode 132, a capacitor 134, one or more LEDs 136, a power switch 138, a current-sensing resistor 140, and two resistors 142 and 144. The system controller 102 includes terminals 104, 106, 108, 110, 112 and 114. The snubber circuit 116 includes a resistor 120, a capacitor 122 and a diode 124. The transformer 118 includes a primary winding 126, a secondary winding 128 and an auxiliary winding 130. For example, the power switch 138 is a transistor.
If the switch 138 is closed (e.g., on), a primary current 148 flows through the primary winding 126, the switch 138 and the resistor 140, and the transformer 118 stores energy. The resistor 140 generates a current-sensing signal 150 which is detected at the terminal 114 (e.g., CS). If the switch 138 is open (e.g., off), the energy stored in the transformer 118 is released to drive the one or more LEDs 136. Information about an output voltage 152 associated with the one or more LEDs 136 is extracted through the auxiliary winding 130. The auxiliary winding 130, together with the resistors 142 and 144, generates a feedback signal 146 that is detected at the terminal 106 (e.g., FB). Based on at least the current-sensing signal 150 and the feedback signal 146, the system controller 102 outputs a gate drive signal 152 through the terminal 112 (e.g., GATE) to drive the switch 138 in order to regulate a current 154 that flows through the one or more LEDs 136.
But the system 100 often has low efficiency in power transfer, and such low efficiency usually results from low efficiency of the transformer 118 and/or energy loss in the snubber circuit 116. Additionally, many peripheral devices of the system 100 may not satisfy certain requirements for the PCB size.
Hence it is highly desirable to improve the techniques of driving LEDs.